1. Technical Field
The present invention related to a semiconductor device including a system LSI etc.
2. Background Arts
Known is a method of changing a well voltage of an element depending on when in a standby status and when in an active status in order to reduce power consumption of the semiconductor device such as the system LSI.
For example, in the case of a NMOS transistor, when a minus voltage is applied to a substrate, a reverse bias occurs between the substrate (p-well) and an n-type source/drain. Therefore, a depletion layer of the substrate at a lower part of a gate spreads, and, as compared with a case of having no reverse bias, an extra gate voltage is needed for inducing the same quantity of channel electric charge (electrons), resulting in a rise in threshold voltage of the NMOS transistor. As a consequence of this, an off-current flowing to between the source and the drain when the gate voltage is off, is restrained. In the case of a PMOS transistor, conversely, when a plus voltage is applied to the substrate (i.e., n-well), the off-current is similarly restrained.
An element to be driven in a way that changes a well voltage of the element depending on when in the standby status and when in the active status, will hereinafter be termed a well voltage variable element. Further, a transistor to be driven in a way that thus changes the well voltage is called a well voltage variable transistor.
Normally, the well voltage (substrate voltage) is controlled per circuit block. Namely, the well voltage is controlled by distinguishing between the standby status and the active status for every circuit block. Hence, it follows that the well voltages of a plurality of transistors are simultaneously controlled.
In the techniques described above, if the voltage supply terminal is distanced away from the well voltage variable element, influence of elements existing on the substrate surface along a route therebetween can not be ignored. Namely, the well voltages fluctuate due to changes of signals of respective components (a gate, a source, a drain, etc) of these elements. This might cause a malfunction due to a fluctuation of a characteristic of the well voltage variable element.